1. Field of the Invention
The present invention relates to a receiver and communication method employing a multi-carrier transmission method utilizing real coefficient wavelet filter banks, that is, the so-called digital wavelet multi-carrier (DWMC) transmission method.
2. Description of Related Art
A multi-carrier transmission method utilizing orthogonal frequency division multiplexing (OFDM) offers wideband communication in the “Digital Terrestrial Television Broadcasting” service in Japan or in a wireless local area network (LAN) system utilizing a standard such as IEEE 802.11a/g. The fast Fourier transform (FFT), which is a kind of a complex filter bank, is usually utilized as a digital modulation/demodulation method in order to provide the multi-carrier transmission.
Instead of the FFT, the present inventors have introduced a new digital modulation/demodulation method utilizing the DWMC transmission method, for example, as shown in U.S. Patent Publication US2003/156014 A1. Synthesizing a plurality of digital modulated waveforms in real coefficient wavelet filter banks produces a transmission signal in the DWMC transmission method. Pulse amplitude modulation (PAM) is used as a method for modulating each carrier.
A DWMC data transmission method will be described with reference to FIGS. 9 to 13.
As shown in FIG. 9, each subcarrier has an impulse response, and impulse responses of each subcarrier are transmitted in an overlapping relationship with each other among a plurality of subcarriers. As shown in FIG. 10, each transmission symbol is formed by a time waveform that is a combination of impulse responses of a plurality of subcarriers. FIG. 11 shows an example of amplitude spectrum. A transmission frame is formed by several tens to several hundreds of transmission symbols shown in FIG. 10 according to the DWMC transmission method. A configuration example of a DWMC transmission frame is shown in FIG. 12. The DWMC transmission frame includes an information data symbol (SB1) for information data transmission and a preamble symbol (SB2) for symbol synchronization, equalization or the like.
In FIG. 13, a communication apparatus introduced by the inventors comprises a receiver 199 and a transmitter 299. The receiver 199 comprises an A/D (analog-to-digital) converter 110 for converting analog data to digital data, a wavelet transformer 120 for performing discrete wavelet transform (DWT), a parallel-to-serial (P/S) converter 130 for converting parallel data to serial data and a decision unit 140 for deciding a kind of receiving signal. The transmitter 299 comprises a symbol mapper 210 for converting bit data to symbol data and performing symbol mapping, a serial to parallel (S/P) converter 220 for converting serial data to parallel data, an inverse wavelet transformer 230 for performing inverse discrete wavelet transform (IDWT) and a digital-to-analog (D/A) converter 240.
An operation of the communication apparatus will now be described. First, in the transmitter 299 the symbol mapper 210 converts bit data of transmit data to symbol data, then performs symbol mapping in accordance with the symbol data, and outputs serial data. Here, PAM is used for symbol mapping. The S/P converter 220 converts the serial data to parallel data and provides a real number (Di, i=1 to M, where M is an integer) to symbol data per every subcarrier. The inverse wavelet transformer 230 performs the IDWT of the Di onto a time axis. Thereby, a sampling value of waveform on a time axis is generated, and a series of sampling values, which represent transmission symbol, are generated. The D/A converter 240 converts the series of sampling values to an analog base-band signal waveform. Then, the transmitter 299 transmits the analog base-band signal waveform to the receiver 199. In addition, the number of the sampling values on the time axis, which are generated by the IDWT, is usually the n-th power of 2 (where n is a positive integer).
Next, in the receiver 199, the A/D converter 110 samples the analog base-band signal waveform with the same sampling rate as that in the transmitter 299, and obtains a series of sampling values. The wavelet transformer 120 performs wavelet transform of the series of sampling values on a frequency axis. The P/S converter 130 converts parallel data to serial data. The decision unit 140 calculates an amplitude value of each subcarrier, and decides what kind of signal the received signal is.
A conventional configuration of the wavelet converter 120 is shown in FIG. 14. As the wavelet converter 120, the use of cosine modulated filter banks (CMFB) utilizing extended lapped transform (ELT) is well-known in the art, as shown in “Signal Processing with Lapped Transforms,” H. S. Malvar, Artech House, 1992, and “Multirate Systems and Filter Banks,” P. P. Vaidyanathan, Prentice-Hall, 1992.
As shown in FIG. 14, the wavelet converter 120 comprises a waveform register 121, butterfly operation units 122a and 122b (which may be, for example, Cooley-Tuke type FFT algorithm processing units), registers 123a, 123b and 123c, and a discrete cosine transformer (DCT) 124. In addition, FIG. 14 shows an example of the wavelet transformer 120 that the number of filter banks is equal to 4 and an overlapping factor of ELT is equal to 2. Furthermore, a filter length(L) of the wavelet transformer 120 is equal to NM, where N is equal to 2K.
The waveform register 121 stores a received waveform for one symbol in accordance with the series of sampling values. The butterfly operation units 122a and 122b perform a butterfly operation for M inputted signals based on parameters of the butterfly operation. Each of the registers 123a, 123b and 123c delays an inputted signal by one symbol, and outputs the one symbol-delayed signal. The DCT 124 provides an output signal representing a discrete cosine transformed input signal as parallel data.
Next, an operation of the wavelet transformer 120 will be described. A detailed description regarding a general operation of a wavelet transformer utilizing butterflies is provided in the H. S. Malvar reference cited above. Now, the waveform register 121 stores a received waveform of Nth symbol, and outputs the Nth waveform. The butterfly operation unit 122a receives the Nth waveform, performs a butterfly operation on the Nth waveform, and outputs a result of the operation for a received signal of the Nth symbol.
The register 123a delays the output signal from the butterfly operation unit 122a by one symbol, and outputs the one-symbol-delayed signal. Therefore, the output from the register 123a changes from operation result of nth symbol to operation result of (N−1)th symbol. The register 123b delays the output signal from the register 123a by one symbol, and outputs the one-symbol-delayed signal. Therefore, the output from the register 123b changes from operation result of (N−1)th symbol to operation result of (N−2)th symbol.
The butterfly operation unit 122b performs a butterfly operation on the output from the register 123b (operation result of (N−2)th symbol) and the output from the butterfly operation unit 122a (operation result of Nth symbol), and outputs operation results of both the Nth symbol and the (N−2)th symbol.
The register 123c delays outputted signals for both the Nth symbol and the (N−2)th symbol from the unit 122b by one symbol, and outputs the one-symbol-delayed signals. Therefore, the output from the register 123c changes from operation result of both nth and (N−2)th symbols to operation result of both (N−1)th and (N−3)th symbols.
The DCT 124 performs an orthogonal transform between the operation result of both the nth and the (N−2)th symbols, and also performs an orthogonal transform between the operation result of both the (N−1)th and the (N−3)th symbols.
The wavelet transformer 120 shown in FIG. 14 operates to demodulate by operating to receive by one symbol and then to delay the received waveform of the symbol by at most three symbols (=N−1).
Therefore, if symbol timing deviation occurs, the deviation will be detected four symbols after the deviation occurs. Furthermore, in order to demodulate data at the timing after the deviation occurred, the wavelet transformer 120 needs to restart to get data at the right timing. Getting data takes additional time corresponding to four symbols. The conventional wavelet transformer 120 needs to perform its operation corresponding to eight symbols in order to respond to the timing deviation. Therefore, it is difficult for the conventional wavelet transformer 120 to quickly respond to a symbol timing deviation.